The present invention relates generally to a method and apparatus utilized in hydrocarbon exploration. More specifically, the present invention relates to the utilization of acoustic sources and receivers to determine acoustic properties of geologic formations as a logging tool traverses them, be it a wireline logging tool and/or a logging-while-drilling (LWD) tool. More particularly, the present invention is directed to methods of, and apparatus for, adaptive equalization of receivers to reduce and/or substantially eliminate sensitivity mismatch.
Geologists and geophysicists are interested in the characteristics of the formations encountered by a drill bit as the drill bit is drilling a well for the production of hydrocarbons from the earth. Such information is useful in determining the correctness of the geophysical data used to choose the drilling location and in choosing subsequent drilling locations. In horizontal drilling, such information can be useful in determining the location of the drill bit and the direction that drilling should follow.
Such information can be derived in a number of ways. For example, cuttings from the mud returned from the drill bit location can be analyzed and/or a core can be bored along the entire length of the borehole. Alternatively, the drill bit can be withdrawn from the borehole and a “wireline logging tool” can be lowered into the borehole to take measurements. In still another approach, called “measurement-while-drilling” (MWD) and/or “logging-while-drilling” (LWD), tools make measurements in the borehole while the drill bit is working. There are a wide variety of logging tools, including resistivity tools, density tools, sonic and/or acoustic tools, and imaging tools, and the like.
An acoustic logging tool collects acoustic data regarding underground formations. One of the purposes of such a tool is to measure the “interval transit time” or the amount of time required for acoustic energy to travel a unit distance in a formation. In simple terms, this is accomplished by transmitting acoustic energy into the formation at one location and measuring the time that it takes for the acoustic energy to travel to a second location and/or past several locations. The measurement is complicated by the fact that the tool is roughly in the middle of a borehole of unknown diameter and is surrounded by mud. Furthermore, the formation along the borehole may have been disturbed by the action of the drill bit and may no longer have the same acoustic characteristics as the undisturbed formation.
Acoustic well logging is a well-developed art, and details of acoustic logging tools and techniques are set forth in A. Kurkjian, et al., “Slowness Estimation from Sonic Logging Waveforms,” Geoexploration, Vol. 277, pp. 215-256 (1991); C. F. Morris et al., “A New Sonic Array Tool for Full Waveform Logging,” SPE-13285, Society of Petroleum Engineers (1984); A. R. Harrison et al., “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source,” SPE 20557, pp. 267-282 (September 1990); and C. V. Kimball and T. L. Marzetta, “Semblance Processing of Borehole Acoustic Array Data,” Geophysics, Vol. 49, pp. 274-281 (March 1984), all of which are hereby incorporated by reference herein.
An acoustic logging tool typically includes an acoustic source (transmitter), and a set of receivers that are spaced several inches or feet apart. An acoustic signal is transmitted by the acoustic source and received at the receivers of the borehole tool that are spaced apart from the acoustic source. Measurements are repeated every few inches as the tool passes along the borehole.
The acoustic signal from the acoustic source travels through the formation adjacent the borehole to the receiver array, and the arrival times, and perhaps other characteristics of the receiver responses, are recorded. Typically, compressional wave (P-wave), shear wave (S-wave), and Stoneley wave arrivals and waveforms are detected by the receivers and are processed. The processing of the data is often performed on the surface, although it may also be performed real-time in the tool itself. Regardless, the information that is recorded is typically used to find formation characteristics such as formation slowness (the inverse of acoustic speed) and anisotropy, from which pore pressure, porosity, and other formation property determinations can be made. With some tools, the acoustic signals may even be used to image the formation.
Acoustic logging tools are used for both wireline logging and logging-while-drilling (LWD) applications. In wireline logging, a probe, or “sonde,” housing multiple logging tools is lowered into the borehole after some or all of the well has been drilled. The sonde is attached to a conductive wireline that carries power from the surface to the tools in the sonde, and that carries telemetry information to the surface. The sonde may be transported through the borehole by the wireline, or a separate transport means may be provided. For example, in “pipe-conveyed” logging, the sonde is mounted on a tubing string. The rigidity of the tubing string allows the sonde to be transported through highly deviated and horizontal boreholes.
The problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed or “tripped” from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and stacked on the platform (if offshore). Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there is a strong incentive to minimize the number of wireline logging trips. One way to do this involves collection of data during the drilling process. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or MWD. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging-while-drilling” techniques, or LWD. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term logging-while-drilling (LWD) will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Logging-while-drilling (LWD) tools are generally located as close to the drill bit as possible, so as to minimize the delay between reaching a formation and measuring the properties of the formation. When implemented as logging-while-drilling (LWD) tools, acoustic logging tools must overcome a number of obstacles to perform successfully. These obstacles include drilling noise, and acoustic properties of the thick tool body. Accordingly, acoustic logging tools in both wireline and logging-while-drilling (LWD) applications have challenges to overcome.
Furthermore, another well-known problem in acoustic well logging, especially for the acquisition of higher order modes, is that receiver matching is important in producing correct results. However, proper and complete receiver matching is very hard to achieve in downhole conditions.